The Advanced Configuration and Power Interface (ACPI) is an open industry specification that enables operating system-directed configuration, power management, and thermal management of mobile, desktop, and server platforms.
The ACPI defines six power states for computing devices: S0, S1, S2, S3, S4, and S5, as follows:
S0: the run state. In this state, the computing device is fully running.
S1: the suspend state. In this state, a central processing unit (CPU) suspends activity but retains its contexts.
S2/S3: sleep states. In these states, memory contexts are held, but CPU contexts are lost. The differences between S2 and S3 are in CPU re-initialization performed by firmware and device re-initialization.
S4: a sleep state in which contexts are saved to disk. The contexts are restored upon the return to S0. This is identical to a soft-off for hardware. This state can be implemented by either the operating system (OS) or firmware.
S5: the soft-off state. All activity stops and all contexts are lost.
In some computing devices that implement ACPI, states S1 and S2 are not employed. As a result, a given computing device is either in a sleep state S3/S4/S5 or in an active or run state S0.